Dual-Fermi-level hydrogel SERS substrates for synergistic chemical and physical enhancement

Surface enhanced Raman scattering (SERS) hydrogel substrates face limitations in chemical enhancement (CM) due to the single-metal Fermi level, restricting analyte detection. Herein, negatively charged silver nanoparticles (AgNPs) and positively charged gold nanoparticles (AuNPs) were engineered into bimetallic hydrogel substrates (AgNPs@AuNPs-chips). The obtained AgNPs@AuNPs-chips exhibited a strong localized surface plasmon resonance (LSPR) peak matching the 785 nm laser for significant electromagnetic (EM) enhancement, while the dual Fermi levels (-4.26 eV for Ag, -5.10 eV for Au) enabled more photo-induced charge transfer (PICT) pathways for CM. Compared to monometallic AgNPs-chips with similar LSPR properties, the bimetallic chips demonstrated better synergistic EM/CM effects for some molecules. When applied to detect maleic hydrazide (MH), the chips achieved a limit of quantitation of 10 ppb, surpassing prior AgNPs-chip methods (100 ppb). Successful quantitative analysis of MH in real-world samples with good recovery demonstrated the substrate's practicality. This work pioneers dual-Fermi-level colloidal interfacial design, expanding SERS applicability in food safety.